Detection of clinically unexpected malignant and premalignant tumors with whole-body FDG PET: histopathologic comparison.

Purpose: To determine the clinical importance and malignant potential of unexpected abnormal foci of hypermetabolism at fluorodeoxyglucose (FDG) positron emission tomography (PET) performed for evaluation of malignancy.

Materials And Methods: A total of 1,750 FDG PET scans were obtained to evaluate a variety of known or suspected malignancies. Each scan was evaluated for abnormal unexpected hypermetabolism based on unusual location (ie, foci that did not conform to the usual distribution of metastases given the primary tumor for which the PET scan was requested) and discrete focal nature of an abnormality. Unexpected findings were followed by pathologic confirmation and were considered clinically important if the final pathologic diagnosis was cancerous, precancerous, or noncancerous but had the potential for local destruction or systemic physiologic effects.

Results: On the basis of the normal spread pattern of the primary lesion, 58 abnormal unexpected foci of hypermetabolism were identified in 53 patients. Forty-five of these abnormalities were followed up with computed tomography (CT), magnetic resonance imaging, and/or mammography, and 42 had subsequent tissue confirmation at endoscopic, CT-guided, or surgical biopsy. Of 42 histopathologically confirmed abnormalities, 30 (71%) were either malignant or premalignant tumors that differed from the cancer for which the patient was originally scanned. Nine other suspicious abnormal foci proved benign and three represented false-positive findings, with no abnormal findings at endoscopy. Three of nine nonmalignant lesions were considered clinically important because of the potential for local destruction and/or systemic effects.

Conclusion: The identification of unexpected foci of hypermetabolism at whole-body FDG PET may signal the presence of tumors that are unrelated to the neoplasm for which the patient was scanned. Findings of this study emphasize the need for follow-up of these abnormalities because the majority represent either malignant or premalignant neoplasms, which were not clinically apparent.